1. Field of the Invention
The present invention relates to disk drives. In particular, the present invention relates to a disk drive disabling a back electromotive force (BEMF) detection window to reduce acoustic noise while using wedge spindle speed control.
2. Description of the Prior Art
A disk drive typically comprises one or more disks rotated by a spindle motor while heads are actuated radially over the disk surfaces. Each disk surface comprises a number of radially spaced, concentric tracks, where each track is divided into a number of data sectors. A number of embedded servo wedges are also written on each disk surface which facilitate seeking the head and maintaining the head over the centerline of a target track during read and write operations. The disks are rotated at a constant angular velocity (CAV) while varying the data rate from an inner diameter zone to an outer diameter zone to maximize the recording density.
In order to achieve accurate reproduction, it is important to maintain the spindle motor “at speed” while writing data to and reading the data form the disks. To this end, prior art disk drives typically control the spindle speed by monitoring zero crossings in the BEMF voltage generated by the un-energized winding within the spindle motor. The BEMF zero crossings are also used to implement a phase-lock-loop (PLL) commutation clock for clocking a commutation sequencer to energize the appropriate windings at the appropriate time. However, the bandwidth of a spindle speed control loop using BEMF voltage as feedback may be insufficient for a desired recording density or vibration tolerance. In addition, monitoring the BEMF voltage generated by the un-energized windings can cause spike currents creating undesirable acoustic noise.
U.S. Pat. No. 6,067,202 suggests to measure the time between servo sector pulses coincident with detecting each servo wedge, and to generate a speed error by comparing the measured time to a reference time corresponding to the desired spindle speed. Since the servo sector pulses occur more frequently than zero crossings in the BEMF voltage, the bandwidth of the spindle speed control loop increases, allowing for higher recording densities and/or improved vibration tolerance. However, the '202 patent discloses very little implementation details for a spindle speed control loop using servo sector pulses as feedback, and in particular, no implementation details on certain drawbacks associated with the BEMF speed control.
The spindle motor is typically driven in a PWM mode for power efficiency. A BEMF speed controller generates a PWM signal in order to “energize” selected windings of the spindle motor with a chopping current. The BEMF speed controller processes a BEMF speed error measured by detecting zero crossings in the back EMF voltage generated by an un-energized winding. The PWM signal is typically disabled (e.g., held high) during a BEMF detection window to pause the chopping action so that the BEMF zero crossings can be detected accurately. However, disabling the PWM signal causes current transients in the spindle motor windings, which results in acoustic noise, torque/speed jitter, and disk vibration.
There is, therefore, a need to reduce acoustic noise in a disk drive caused by disabling the PWM signal driving the spindle motor during a BEMF detection window.